On most military aircraft, liquid oxygen is needed to supplement or enrich the atmosphere at flying altitudes. Liquid oxygen is stored in cryogenic containers.
It has been found that when a cryogenic container, such as shown in U.S. Pat. No. 3,043,466, is filled with liquid oxygen and allowed to warm up, the temperature of the liquid on the top surface rises more rapidly than the remaining mass of the stored liquid oxygen. Because of this temperature difference, stratified layers of oxygen at different temperatures and density are present in the stored liquid oxygen. The layer of liquid oxygen having the temperature and lowest density is located on the top of the liquid oxygen. Thus, even though the stored liquid oxygen is stratified, it is in a fairly stable internal condition. Unfortunately as the temperature of the liquid oxygen rises, the volume of the liquid oxygen expands. The expansion of the liquid oxygen covers a vent port causing increase in the internal pressure in the cryogenic container. After the internal pressure reaches a predetermined value, a relief valve such as shown in U.S. Pat. No. 3,707,078 opens and a portion of the liquid oxygen is vented through a relief port to the atmosphere. Once the liquid oxygen in a cryogenic container has been warmed to a point where venting is required, a rapid reduction in the retained volume takes place. The stand-by time for most cryogenic containers is about 48 hours. Therefore, if an aircraft is on the ground for longer than two days without being serviced, each cryogenic container must be refilled to a preset volume to assure the aircraft personnel of sufficient oxygen to operate the aircraft.